The air conditioner and taillights are just two examples of the many electrical systems in your car. When something goes wrong, it can be challenging to pinpoint the issue with one of those parts, but when it comes to the electronics of a car, there’s a good probability the issue is connected to a relay.

a relay

Relays manage a number of electrical components in your car. They act as switches, allowing one circuit to control another despite amperage differences. Knowing where the issue is when they break down is crucial for a secure repair that doesn’t harm the car. Fortunately, testing a relay is a job that even inexperienced mechanics can complete.

Well, in this article, we’ll walk you through how to test a relay. Nonetheless, you’ll get to know the answer to the following questions:

Read more: Understanding an automotive relay

Read more: How to test a car battery with a multimeter

Ok, let’s dive in!


What is a relay?

A relay is an electrically operated switch. A set of functioning contact terminals and a set of input terminals for one or more control signals make up the device. Any number of contacts, including make contacts, break contacts, and combinations of both, may be present on the switch.

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What is the function of relays?

Relays are electronic parts that exchange high voltages or currents with a lower voltage or current. They function as a switch that is controlled by an electrical signal. The relay opens or closes a circuit in response to signals, allowing or preventing current flow.

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Where are relays used?

When multiple circuits need to be controlled by a single signal or when a circuit has to be controlled by a separate low-power signal, relays are used. Initially, relays served as signal repeaters in long-distance telegraph lines, reviving the signal coming in from one circuit by relaying it on another. Early computers and telephone exchanges both made heavy use of relays to carry out logical processes.

how to test a relay

The usage of relays is common in cars, as well as in maritime and aviation applications. They are typically employed to enable the control of larger amperage circuits by low amperage circuits. Relays serve as switches that enable the low amperage circuit to turn on or off the high amperage circuit.

As an illustration, consider the headlights of your car. The amperage could surpass the switch’s capacity if you attached the lights directly to it, which could lead to cables melting and possibly even a fire in some circumstances. Relays can also manage many systems simultaneously, such as turning on the headlights in conjunction with the radio or extending the antenna in conjunction with the windshield wipers.

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What are some safety tips when working with relays?

Although relay testing is an easy process, safety should always come first. When working with relays, it’s a good idea to consider the following:

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How do you test a relay?

There is no need for a whole kit to check the relays in your car because it is a straightforward procedure. You’ll need these items to check for defective relays: a high-impedance test light, jumper wires, Ohmmeter (commonly known as a digital multimeter or DMM), a replacement relay, and a vehicle service manual (not really necessary but highly recommended).

Here are steps to guide you on how to test a relay:

Find the relay that needs testing. Depending on what it controls, it can be under the dash or in the engine compartment. Check the electrical chapter and wiring diagram in your service manual if you’re unclear about the location.

Replace the relay with a known-good replacement to test it quickly and easily. The drawback is that the second relay can catch fire due to the identical issue that destroyed the first one. Moreover, it reaches into your wallet. While the relay is disconnected, check and clean the connectors.

Set an Ohms multimeter to Ohms. Measure resistance by touching the leads across the electromagnet coil pins. Between 50 and 120 ohms is OK. It’s time for a replacement relay if the relay is out of range or is open due to a faulty electromagnet coil winding.

Keep the multimeter set to continuity or ohms. Cross the switch pins with the leads. Relays that are normally open should display open or OL.

Across the pins, connect a 9–12 V battery to power the electromagnet coil. As the electromagnet coil energizes and shuts the switch, there should be an audible “click” from the relay. On this 4-pin relay, polarity is unimportant, but on diode relays, it is crucial.

Jump the battery positive to one switch terminal while the coil is still active. Connect a test light between the ground and the other switch terminal. The current should flow to the test light, causing it to shine. Remove the battery-positive jumper. The test light ought to turn on.

Check the voltage of the relay at the switch. A voltage drop may result from poor contact points. Remove the test light, turn the multimeter to DC volts, and then touch the leads across the test light connectors or switch pins as indicated. The voltage of the battery should match the reading.

To ensure proper resistance, test the switch. The positive jumper wire should be disconnected. Energize the electromagnet coil. Measure the resistance between the switch pins using the multimeter set to ohms. A usually closed relay should measure open, or OL, while an energized normally open relay should measure close to zero ohms.

Test Your Relay In One Minute By Watching This Video:

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What causes relay failure?

Leaky contacts, cracked contacts, or significant size and position displacement are the main negatives. This will have an impact on the relay’s contact dependability. Typically, contact overheating, wear, and welding are the problems of contact parts.

Can bad relay drain battery?

The ignition relay is essentially a switch for the power to the ignition system’s electrical components. Making sure electricity gets from the battery to the various components that require power is its responsibility. A defective relay will drain your battery and make it challenging to turn the ignition.

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Can you test a main relay?

It’s rather easy to determine if your main relay is impacted. Just start the car and see if it keeps running. If so, the primary relay is working properly. The primary relay can be affected if the engine cuts out.

What type of tests are done on relays?

Testing of the voltage and current transformers. checking the settings for the relay alarm and trip. both initial and follow-up injection testing.

How do relays work?

An electromagnet drives an electrical switch known as a relay. A little current passing through the relay’s wire coil produces a magnetic field that attracts a metal switch, or contact, to the area of the coil. By closing the circuit, this contact permits a greater current to flow through it.

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Is a relay bad if it clicks?

No, not always. The electromagnetic coil inside a relay is what activates and deactivates to cause the switch to open and close, hence a clicking sound from the relay might be typical. The relay may need to be changed if the clicking sound is abnormal, such as if it is louder than usual or if it is accompanied by additional symptoms like a failure to start or intermittent operation.

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How do you test an overload relay?

To test an overload relay, you will need a multimeter. Here are the steps to follow:

  1. Disconnect the power supply to the motor and remove the wires from the overload relay.
  2. Set your multimeter to the continuity mode, which will allow you to test for a complete circuit.
  3. Touch one probe of the multimeter to one of the overload relay terminals and the other probe to another terminal. There should be no continuity between these two terminals.
  4. Now, touch one probe to the first terminal and the other probe to the terminal where the motor wire was connected. There should be continuity between these two terminals.
  5. Repeat this process for the other terminal where the motor wire was connected.
  6. If you get a reading of continuity in step 3, then the overload relay is faulty and needs to be replaced.

Note that this is a basic test and may not detect all faults with an overload relay. If you suspect that there is an issue with the relay, it’s best to consult a professional electrician or technician for a more comprehensive test.

Does relay need battery?

No, a battery is not needed for a relay to function. A relay is an electromechanical device that controls the flow of current in an electrical circuit by operating a switch with the help of an electromagnetic coil. The coil doesn’t need a battery to work because it is powered by a tiny current, often via a control circuit. However, certain relays could come equipped with LED indicators or other features that need a tiny battery or other power supply to function.

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Can you test a relay with a 9v battery?

Yes, you can test a relay with a 9V battery. Here are the steps to follow:

  1. Disconnect the relay from the circuit it’s in, and remove any wires connected to it.
  2. Take a 9V battery and connect a wire to the positive terminal of the battery.
  3. Touch the other end of the wire to the input pin of the relay (the pin that activates the electromagnet).
  4. Take another wire and connect it to the negative terminal of the battery.
  5. Touch the other end of this wire to the ground pin of the relay (the pin that is connected to the metal frame of the relay).
  6. You should hear a clicking sound, and the switch inside the relay should move from one position to another.
  7. If you don’t hear a clicking sound or if the switch doesn’t move, then the relay may be faulty and may need to be replaced.

Note that this is a basic test and may not detect all faults with a relay. If you suspect that there is an issue with the relay, it’s best to consult a professional electrician or technician for a more comprehensive test.

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What makes a relay activate?

When small current flows through a relay’s wire coil, it generates a magnetic field that attracts a metal switch, or contact, to it, activating the relay. A control circuit, such as a switch, a sensor, or a microcontroller, can regulate the relay’s activation. A tiny current passes through the coil of the relay when the control circuit delivers a signal to it, activating it and allowing a greater current to flow through the switch. The control of numerous electrical appliances, including heaters, lights, and motors, can be accomplished by using this bigger current.

What causes a relay to activate?

An electrical signal that energizes the coil of wire inside the relay activates the relay. This electrical signal may originate from a switch, a sensor, or a microcontroller, among other things. An electrically charged wire coil produces a magnetic field that attracts a metal switch or contacts to it. By closing the circuit, this contact permits a greater current to flow through it. Depending on the layout and function of the electrical system it is a part of, the particular reason why the relay activates will vary.

What causes a relay to trip?

A relay can trip or “open” due to a variety of reasons, such as:

In some cases, a tripped relay can be reset by turning off the power and allowing the relay to cool down before turning the power back on. However, if the relay continues to trip or fails to operate properly, it may need to be replaced or repaired by a qualified technician.

That is all for this article, where we looked at how to test a relay. Nonetheless, the answers to the following questions was discussed:

We hope it was helpful. If so, kindly share it with others. Thanks for reading; see you around!